1. Field of the Invention
The present invention relates to a method and apparatus for measuring the output of a sensor such as an optical sensor which receives a physical quantity such as light to be measured and outputs a measurement signal having a time width or time duration as a function of the size of the physical quantity, and more particularly to a method and apparatus suitable for measuring the intensity of lights received by an array of optical sensors incorporated in an autofocus camera.
2. Description of the Prior Art
Light, sound or the like, which is a physical quantity to be measured, has a very wide dynamic range, for example, of approximately 1:10.sup.6 and therefore a sensor for sensing these types of physical quantity should preferably have a measurement range that can discriminate the size of the physical quantity over the entire dynamic range thereof.
However, using the amplitude of an electrical signal from the sensor to represent the size of the physical quantity only permits discriminating the physical quantity within a dynamic range of 1:10.sup.3 at the best; therefore, it is common that the electrical signal is converted into another electrical signal wherein the time width or time duration of the signal varies as a function of the size of the physical quantity, and this converted signal is then output from the sensor. For an optical sensor, it is advantageous to employ a so-called charge storage type for converting the intensity of a light to be measured into the time width. A circuit example of such an optical sensor is shown in FIG. 5, and operational waveforms at various points in the sensor in FIG. 6.
FIG. 5 shows an optical sensor 1, which receives a light or a physical quantity L through a photodiode 1a. Photodiode 1a is, for example, of an optoelectric conductive type. When initializing the measurement of the light intensity, a transistor 1b in series with photodiode 1a is caused to be "ON" by a reset pulse RP shown in FIG. 6(a), applying a voltage V shown in FIG. 5 in a reverse polarity across photodiode 1a to charge the junction capacitance of photodiode 1a to voltage V. At this time, a voltage v at the connecting node between photodiode 1a and transistor 1b is equal to a ground potential E as shown in FIG. 6(b).
When photodiode 1a generates a photocurrent proportional to the size of physical quantity L, a capacitor C shown in FIG. 5 is caused to discharge by this photocurrent, and correspondingly, voltage v rises linearly with a time t, as shown in FIG. 6(b).
An inverter 1c is applied with voltage v, and an output S thereof rises to a logic level "1" at the same time as reset pulse RP rises to the same logic level, as shown in FIG. 6(c). Output S falls to a logic level "0", when voltage v reaches a threshold value Vth of inverter 1c. This output S is the measurement signal from sensor 1 and we have a functional relationship .tau..alpha. 1/.lambda., where .tau. is the time width or time duration of measurement signal S and .lambda. is the size or intensity of physical quantity L to be measured.
By employing sensor 1 of, for example, a charge storage type and converting the size of physical quantity L to be measured into time width .tau., measurement signal S can be obtained in terms of time width .tau. exactly indicative of size .lambda. of physical quantity L to be measured, even when the physical quantity to be measured has a very wide dynamic range.
Expressing time width .tau. directly in a numerical value can cause the value to vary over a dynamic range as wide as 1:10.sup.6 if physical quantity L to be measured has a dynamic range of 1:10.sup.6 as stated above. Thus the numerical value of measurement output S may conveniently be converted into a logarithmic value, for subsequent manipulation thereof. For example, if the physical quantity to be measured is a light, it is expressed in a logarithmic so-called EV value, which is commonly used as a means for indicating the light intensity.
As shown in FIGS. 7 and 8, in Japanese Preliminary Patent Publication No. 62-204184, Applicant of the present invention has proposed a method for converting time width .tau., which is indicated by measurement signal S into a digital value which indicates time width .tau. in a logarithmic value.
FIG. 7 shows a schematic block diagram of a circuit incorporating such a method. A clock pulse generating circuit 2 generates clock pulses EVC, which time period gradationally increases at a common ratio. An AND gate 3 is provided for clock pulses EVC as well as measurement signal S from the sensor 1, with time width .tau. as shown in FIG. 8(a), from the sensor 1. The output of AND gate 3 provides count pulses CT shown in FIG. 8(c) to a counter 4. A digital value DT, which is the count value output of counter 4, represents in a logarithmic relationship the size of physical quantity L to be measured. Such physical quantity, for example, can be a light received by sensor 1, from which the aforementioned EV value can easily be known, for example. Even when a large number of digital values are involved, relating to a sensor array with a large number of optical sensors therein, for example, a small number of digits can still indicate accurately the pattern or contrast of the image received by the optical sensor.
However, the variable time period clock pulses of the prior art, used in the above-mentioned method, require that the ratio between the period of adjacent pulses is selected not to be an integer but a non-integer value very close to 1 to accurately represent the size of the physical quantity to be measured in a digital value. For this reason, clock pulse generating circuit 2 shown in FIG. 7 must be formed by combining a divider with a switching means for providing an input to the divider and an output therefrom. This creates a problem in that the circuit arrangement becomes complicated.
Moreover, although the digital value, as a measurement obtained by means of the clock pulse, is useful in many applications because it is in a logarithmic relationship with the size of the physical quantity to be measured, there are some situations in that a functional relationship other than logarithmic relationship is preferred. In these situations clock pulses having a time period varying at a non-common ratio must be generated to accommodate the functional relationship, further causing the circuit arrangement of the clock pulse generating circuit extremely complicated.